It is probably the clutch. It jerks forward right? I would try unscrewing the idle screw until you can hold the rear wheel without much force to see if the engine can't run that slow. It could be that the engine won't idle that low because of a dirty carb or bad ignition points or spark plug. You could also try gradually putting on the back brake and see how low the engine will run and if the clutch eventually slips or just stalls out.